Two types of viruses have been examined for their role in the etiology of bone marow failure in animals and man. The first, the Parvoviridae, are the smallest DNA containing animal viruses. The B19 human parvovirus causes selective erythroid aplasia in individuals with underlying hemolysis. Feline panleukopenia virus is a common agent of cat disease and capable of causing a true pancytopenia and bone marrow aplasia. The human parvovirus was discovered only 10 years ago and a major stumbling block to its investigation has been an adequate in vitro cell culture system. Using suspension cultures of erythroid bone marrow cells from patients with sickle cell disease, we have achieved the first propagation of the B19 agent. This virus is highly selective for erythroid in comparison to myeloid progenitor cells. Events associated with the replication, transcription, and protein production of this virus have been elucidated in detail. The feline virus, in contrast, is less selective in vitro, with a strong inhibitory action on the proliferation of both erythroid and myeloid cells. This virus can also be propagated in suspension cultures of cat bone marrow. Studies of the cat retrovirus, feline leukemia virus, have shown that its effect on in vitro colony formation by cat cells is minimal. In animal studies, feline leukemia virus had marginal effects on hematopoiesis. Extensive studies of human patients of a possible role for a retrovirus in human aplastic anemia had failed to reveal consistent reverse transcriptase activity or retroviral particles by electron microscopy.